The present invention relates to a method and apparatus for separating soap from liquor in a filtrate tank. The method and apparatus according to the invention are particularly well suited for treating soap-containing filtrates from digester houses, bleach plants and washer rooms in the chemical pulp processing industry.
Soap appears primarily in the liquor leaving either the digester or the subsequent washing equipment. Such liquor is called weak black liquor and, besides cooking chemicals, it also contains various organic and inorganic materials dissolved in liquor during pulping. It has been estimated that these materials represent about 12 to 18 percent of the black liquor by weight. Part of these materials precipitate on the surface of the liquor, in the form of soap, the maximum density whereof is only a little lower than that of the liquor. The soap density ranges widely, from 0.98 kg/l down to 0.005 kg/l. According to a reference, a constituent is referred to as soap if its density is higher than 0.1 kg/l and as foam if its density is lower than that. When the soap density is at the maximum, there is not much difference between the soap and liquor densities, whereby the major portion of soap lumps, lumps being the form of soap when it is at its heaviest, remains below the liquor surface.
From the digester or the subsequent washing equipment, weak black liquor is discharged into a filtrate tank. Filtrate tanks are so large, their diameters being of the order of 8 to 16 m and volumes of about 400 to 4000 m3, that the retention time of black liquor in the tank is relatively long. This provides the soap with an excellent opportunity to separate from the liquor and to accumulate on the surface thereof. One purpose of the large tank is to serve as a buffer in production fluctuations. Another purpose is to separate both the gas and the foaming soap from the liquor in order to be able to recirculate a substantially gasfree and soapfree black liquor back to the process, either through chemical recovery or in some other appropriate manner. In some cases, soap accumulates on the liquor surface, forming a layer, which may be even dozens of centimeters thick. Depending on mainly the density of soap, but also on the composition (on how much black liquor is contained in the soap), the form and behavior of soap may vary considerably. If soap is relatively free of black liquor, the soap material becomes viscous, sticky, sometimes lumpy and difficult to move. Such material easily clogs pipes, valves and other soap-treating devices. On the one hand, a soapy foam containing plenty of gas requires a large storage volume in order to ensure that foam cannot escape through ventilation ducts out of the filtrate tank. On the other hand, if soap is stored in the filtrate tank for too long, the soap density may increase to such an extent, when liquor separates therefrom, that the soap will sink into the liquor mass and be carried therewith to the evaporators of the recovery plant. Presence of soap would substantially disrupt the operation of the evaporation plant. For these reasons, it is important that soap be discharged from the tank according as it accumulates on the liquor surface.
As an example of soap separation equipment of prior art, a system based on overflowing deserves to be mentioned. In that arrangement, the tank was overflowed, by allowing a soap fraction to flow over the edges of a special chute or the tank, directly into a sewer. It is clear that this is not possible today, for both environmental and economical reasons. On the one hand, soap must not end up in water systems and, on the other hand, it is possible to prepare, e.g., tall-oil from the soap fraction separated in the tank. In slightly more advanced soap separation systems, the wall of the filtrate tank is arranged with outlets. The idea is to discharge the soap fraction through these outlets and then pump it for further treatment. The great variation of the filtrate level in the filtrate tank is, however, problematic, because it makes it impossible for the operator to know the soap level in the tank, even though the tank were provided with outlets at several heights. This is further complicated by the fact that one never knows, outside the tank, how thick the layer of soap is on the black liquor surface. In other words, it may happen that practically mere liquor is pumped out of the tank for further treatment of soap.
In the worst case, soap sinks into the black liquor, whereby it may end up, e.g., in the evaporation plant and more or less disrupt its operation, depending on the type of the evaporation plant. For example, in such evaporator arrangements in which the material to be evaporated flows along the inner surface of evaporator tubes, soap lumps may clog the entire evaporator tube.
Another way of separating soap is to arrange several filtrate tanks in series, one of which is allowed to overflow so that the soap fraction is separated by the overflow and the black liquor is pumped further to another tank. This arrangement is expensive, though, because of both the tanks and the instrumentation needed. In this arrangement, the main role is played by the level adjustment of the first, so-called soap tank, which level is to be optimal in view of the overflow. In other words, the overflow has to be adjusted in such a way that as little as possible of black liquor is entrained with the soap, yet so, that the soap is efficiently separated in the soap tank.
Prior art also involves many xe2x80x9cfreelyxe2x80x9d floating soap separating systems, in which ballast is used in order to cause a soap separation funnel or equivalent to partially submerge in the liquor so that light soap will flow into a recovery pipe from one end thereof. The pipe has to be sufficiently large, though, in order to prevent soap from clogging it. The pipe itself constitutes a problem because it also serves as a float, in most cases even more efficiently than the float itself. As soap then flows into the pipe at a varying speed, which is characteristic of it, the weight of the pipe changes and the pipe starts to sink deeper and deeper into the soap/liquor. To avoid this, the funnel should either be arranged so high up in the soap layer that heavy soap could not enter the funnel, which would result in the soap ending up to the recovery system with the consequences described above, or the position of the funnel should be monitored and ballast adjusted in accordance with the position of the funnel, so that the funnel would always be in a certain depth in relation to the liquor level. Practically, this is however impossible because there is no way of accurately defining the position of the liquor level. The weight of the soap in the funnel or outlet duct, which directly and, in any case, adversely affects the floating depth of the soap separating funnel, is not reckoned with in any of these freely floating soap separating arrangements.
A soap skimmer is disclosed in, e.g., U.S. patent publication 5,137,643. It separates soap from the surface of the soap tank. The operation is based on a soap skimmer being arranged to rest on at least two floats on the surface of liquor in a filtrate tank. One of the floats, i.e., a so-called top float is arranged to float at least partially submerged in a layer of soap and the other, i.e., a so-called bottom float is arranged to float either solely in soap or partially submerged in black liquor. The top and bottom floats are connected with a hanger plate, which maintains a predetermined distance therebetween. A soap inlet opening is arranged in the bottom float, said inlet opening leading into a soap outlet pipe. The outlet pipe is provided with a vacuum effect to draw soap from the surface of black liquor into the pipe. In other words, since the bottom float with the soap outlet is submerged in soap, soap which is drawn by the vacuum effect flows continuously through the outlet opening into the outlet pipe. Furthermore, the above-described floating assembly is hinged to a swing arm, one end of the swing arm being hinged to the bottom of the tank. The swing arm is used for controlling the movement of the floating assembly as the liquor level varies in the tank.
A drawback of all the prior art means described above is that in all of them the outlet funnel, outlet chute or outlet opening stays in place on the surface of black liquor; in some cases it moves transversely only if the level of black liquor changes. Consequently, there is no other way for the soap to flow into the funnel or, more broadly, to the outlet, than from the vicinity of the outlet opening. The longer the distance from the outlet opening, the more certain it is that soap remains for a long time in the tank. This involves a risk that the soap will sink and end up in the recovery system.
Another drawback already discussed above is free buoyancy of the soap separating means, whereby the weight of soap in the outlet funnel or outlet pipe has an influence on the floating depth of the funnel as well as on its capacity to remove soap from the tank.
The method and apparatus according to the present invention eliminates, e.g., the above-described problem so that the soap outlet opening or equivalent and the soap which is floating on the surface of black liquor are brought into a motion relative to each other, so that it is possible to reliably remove the soap from the entire cross-sectional area of the tank without any risk of the soap remaining for too long in the tank. Furthermore, as no floating soap separation device is used in the present invention, uneven flow of soap into the apparatus does not cause any problems.
It is also characteristic of a method and apparatus according to the invention that soap and the soap outlet opening are brought into a motion relative to each other so that the surface layer of the soap is mixed as little as possible. This is achieved by effecting relative motion without substantially touching the surface layer of the soap.
According to one aspect of this invention there is provided a method of separating soap from liquor in a filtrate tank, comprising: (a) Introducing soap-containing liquor into a filtrate tank. (b) Allowing a soap-containing fraction to separate onto the surface of liquor in the filtrate tank. (c) Introducing the soap-containing fraction into a soap separating assembly while effecting a horizontal draw between the soap fraction and the soap separating assembly so that the introduction of soap fraction into the soap separating assembly is intensified. And, (d) discharging the soap-containing fraction from the tank substantially separately from the liquor. Preferably (a) is practiced using as the soap-containing liquor a soap-containing filtrate from a digester house, bleach plant, or washer room in the chemical pulp processing industry.
According to another aspect of the invention there is provided an apparatus for separating soap from liquor in a filtrate tank comprising: a filtrate tank; a device which introduces liquor into the filtrate tank; a device which skims the foam fraction from the liquor surface in the tank; a device which forms a reciprocal horizontal draw between the soap fraction and the soap separating assembly; and means for discharging the soap fraction from the tank.
Other features characteristic of the method and apparatus according to the invention are seen from the appended claims.